Polyurethane elastomers are preferably produced, according to the known casting process from, for example, NCO-prepolymers with glycols as chain-lengthening agents. Particularly favorable properties are obtained using naphthylene diisocyanate or diphenylmethane diisocyanate with butane diol-1,4 (see Kunststoff-Handbuch, Vol. 7, Polyurethanes, Vieweg and Hochtlen, Carl-Hanser-Verlag, Munich, 1966, pages 207-227). In order to achieve a high hardness in the elastomers, the diisocyanate is reacted with the relatively high molecular weight polyhydroxyl compounds in NCO:OH ratios of greater than 2:1 to produce NCO-prepolymers which thus still contain portions of free diisocyanates, in addition to the NCO-prepolymers.
However, it has also been proposed to increase the NCO content of NCO-prepolymers, such as those based on toluene diisocyanate, by adding other, different diisocyanates and then to further react them with chain-lengthening agents (see Japanese Patent Application No. 53,133,298).
As the NCO content increases, the hardness of the elastomer increases, but the casting time of the reaction mixture is also reduced, and casting is practicable only with glycol-lengthening. With aromatic diamines as the chain-lengthening agents, elastomers having improved elastic and thermal properties are, indeed, generally accessible, but they have a higher reactivity. Consequently, the casting time is shortened in many cases to impracticably short times, unless diamines, such as 3,3'-dichloro-4,4'-diamino-diphenylmethane, which have considerably reduced reactivities (but, unfortunately, also high melting points), are used. Even with these diamines, however, the casting time is extremely short (particularly with NCO-enriched prepolymer mixtures) and partly as a result of this, the casting process cannot be carried out on a practical basis.
The reaction of conventional NCO-prepolymers with chain-lengthening agents, such as, for example, diethyl-tolamines, tetra-alkyl-4,4'-diamino-diphenylmethanes or 4-alkyl-3,5-diamino-benzoic acid alkyl esters, is also difficult because of similar short reactivities. Thus, processing with commercially available isocyanates, such as diphenylmethane diisocyanate or naphthylene diisocyanate, in the form of NCO-prepolymers, is only possible with very short, impracticable casting times (i.e., .ltoreq.5 seconds). Large cast parts or comparatively long flow paths are, thus, hardly feasible (see, Example 4, Table 1, herein).
If reduction of the reactivity of these diamines is attempted by diluting them with relatively high molecular weight polyhydroxyl compounds (such as, for example, adipic acid/C.sub.2 -C.sub.6 -diol polyesters), the results are unsatisfactory. A slight increase in the casting time is indeed achieved, but this has to be balanced against a concurrent and disproportionate increase in the demolding time (see, Example 10, Table 2, herein).
It has now been found that NCO-prepolymers, based on toluene diisocyanate, phenylene diisocyanate or hexamethylene diisocyanate with polyester and/or polyether diols and blended with tetra-alkyl-diphenylmethane diisocyanate, then combined with liquid or low-melting aromatic diamines, may easily be processed into cast parts, even in high pressure installations. These diamines may be less reactive diamines used alone, or in the form of solutions of less reactive diamines in relatively high molecular weight polyhydroxyl compounds and/or, surprisingly, even in the form of solutions of reactive, aromatic diamines, such as diphenylmethane-4,4'-diamine, in relatively high molecular weight polyhydroxyl compounds. During processing, a gradual change in hardness may be obtained by varying the quantity of the added diisocyanates. Moreover, the casting times are long enough for large-volume parts to be produced (see Examples 5 to 9, Table 1 and Examples 11 to 13, Table 2, herein) and the parts may be released from the mold after relatively short molding times.
It is surprising that, in spite of the simultaneous use of different diisocyanates in the casting mixture, the mechanical properties of the end products, which are obtained according to the present invention, are outstanding. Normally, the mixing of several different isocyanate components in the polyurea system causes the production of different urea segments which interfere with each other and thus exert a negative influence on the mechanical property spectrum (particularly on the heat stability and the pressure deformation residue) of the end product. However, good values are actually found precisely in the critical characteristics of heat stability and pressure deformation residue in the products obtained according to the instant invention. The reactivity gradation, and thus the processibility, is also very favorably effected.
In contrast to high-melting diamines, such as 3,3'-dichloro-4,4'-diamino-diphenylmethane, which are slow to react, liquid or low-melting chain-lengthening diamines have a moderate reactivity. Examples of these low-melting diamines include diethyl-tolamines or the (liquid) mixtures thereof, or the mixed condensates from, for example, diethyl aniline, diisopropyl aniline and formaldehyde (such as tetra-alkyl-4,4'-diamino-diphenylmethane), which may be easily processed as diamine melts in high pressure installations.
Alkylated diphenylmethane diisocyanate compounds, and the use thereof in polyurethanes, have been known for a long time. Thus, British Pat. No. 852,651 describes the production of tetra-alkylated diphenylmethane diisocyanates and indicates the suitability thereof for the production of polyurethanes. However, experiments have now shown that formulations which are composed exclusively of tetra-alkyl-diphenylmethane diisocyanates produce waxy-brittle bodies during the reaction with aromatic diamines as chain-lengthening agent, which bodies may only be strengthened by an abnormally-long subsequent heating operation (see Table 1, Examples 1 and 2, herein). Due to this peculiarity, a commercial use of the resulting products is impossible, in spite of some useful mechanical properties.
In the method according to the present invention, the production of a brittle, waxy structure is not observed, in spite of the use of tetra-alkyl-diphenylmethane diisocyanate. On the contrary, moldings which have favorable properties are obtained with practicable casting times and short molding times.